1. Field of the Invention
The present invention relates to a power train control apparatus of a vehicle and, more particularly, to slip control of a vehicle equipped with an automatic transmission.
2. Description of Related Art
Preventing the driven wheels from slipping on the pavement to an excessive extent is advisable to produce an efficient driving force for the automobile or to ensure safety by preventing spin-out of the automobile. An excessive amount of slippage of the driven wheels can be prevented by reducing the torque applied to the driven wheels, as such torque becomes a cause of slippage.
A slip control of this kind--which is sometimes called a traction control--is disclosed, for example, in U.S. Pat. Nos. 4,484,280 and 4,583,611 as well as their Japanese counterparts, i.e., Japanese Patent Publication Laid-open Nos. 16,948/1983 and 56,662/1985. The technology disclosed in these two prior patent publications involves a reduction in torque applied to the driven wheels by a decrease in output of the engine itself as well as by the application of a braking force to the driven wheels by the brake. More specifically, U.S. Pat. No. 4,484,280 and its Japanese counterpart, Japanese Patent Publication Laid-open No. 16,948/1983, disclose the technology in which the driven wheels are braked only when the extent of slippage of the driven wheels is small while output of the engine is decreased, in addition to the braking of the driven wheels, when slippage becomes larger. U.S. Pat. No. 4,583,611 and its Japanese counterpart, Japanese Patent Publication Laid-open No. 56,662/1985, disclose the technology which involves braking only the one side of the driven wheels which slips more than the other when the driven wheels slip to different extents while decreasing the output of the engine in addition to the braking of both sides of the driven wheels when both sides slip to a large extent.
Furthermore, there is recently the increasing tendency that the output of the engine is transmitted to the driven wheels through a multistage automatic transmission. Japanese Patent Publication Laid-open No. 176,828/1985 discloses a vehicle equipped with such a multistage automatic transmission, in which the gearing ratio is upshifted to high speed stages in synchronization with the start of the slip control. This system is such that efficiency in slip convergence is enhanced by the reduction in output of the engine or by the application of a braking force because the torque applied to the driven wheels is reduced by upshifting.
Japanese Patent Publication Laid-open No. 106,160/1987 proposes a correction of shift characteristics so as to allow the high speed range to be increased gradually when a non-slip control state is transferred to a slip control state while the shift characteristics corrected is gradually returned to an orginal level when the slip control state is changed to the non-slip control state.
It should be noted that the upshifting or changing of the shift characteristics on condition of the start of the slip control may give rise to or incur the risk of downshifting as the slip control is terminated. The automatic transmission is operated to provide an automatic transmission of rotational power on the basis of predetermed shift characteristics so that the upshifting on condition of the start of the slip control leads to a forcible change of the shift characteristics during ordinary running. Accordingly, when the slip control has been terminated, the shift characteristics are switched back to those employed during ordinary running, so it can be considered that this switching backcauses downshifting. This downshifting again increases the torque applied to the driven wheels. Thus there is room for improvement from the point of view of slip convergence.